In a polyphase electrical machine, the distribution of the machine performance over several smaller frequency converters is often preferred as a result of redundant demands. The clocking of the individual frequency converters is then associated with the occurrence of noise and oscillating torque. To prevent or remove these unwanted noises and oscillating torques, interphase transformer connections are used in d.c. machines. In these interphase transformer connections, two d.c. chopper controllers that are in phase opposition are attached to an interphase transformer. The interphase transformer feeds the armature winding of the d.c. machine through a central tapping. The additionally required volume, weight and cost of the interphase transformer, however, renders it disadvantageous and impractical.
In twelve-pulse converter-fed motors (with six phases), two three-phase windings offset by 30 degrees are provided, each of which is fed by a three-phase frequency converter. The highest possible number of phases m is thereby limited by the still executable minimum toothpitch .tau..sub.N and depends on the pole pitch .tau..sub.p according to the relation .tau..sub.N =.tau..sub.p /m.
Thus, there is a need to provide an electrical machine driven by a pulse-controlled a.c. converter with several frequency converters, that performs without interphase transformers and is not subject to the conditions that limit the highest possible number of phases.